Industrial machines often undergo unforeseen shutdowns and failures, often associated to aspects related to lubrication. Lubricating oil is one of the key components in some of these machines and provides a lot of information regarding the machines condition. It may be interesting to monitor some parameters in lubricating oil, such as particle determination (for example, quantification, classification of size or determination of shape), bubble content in the system or oil degradation based on colour. Oil degradation is a key indicator of oil quality and how it fulfils its lubricating mission. It does not provide information on the machine directly, but indirectly, from the speed of degradation, it is possible to extract information regarding the machine's operation.
European patent application EP2980557A1 discloses a system for inspecting oil, which comprises a cell through which oil flows along a pipe. The system is based on a lighting system having a LED diode, configured to supply a light beam to the flow of oil, and an image capture system situated on the opposite side of the pipe in respect of the lighting system. The image capture system is configured to capture a sequence of images of the oil that flows inside the pipe. The lighting system and the image capture system are disposed within the cell. A processor processes the sequence of images and determines the presence of particles and a value for the oil degradation.
The system disclosed in EP2980557A1 performs as expected when the oil, and therefore the particles comprised therein, travel along the pipe at low speed. However, it has been observed that when the instant velocity of the particles is relatively high, when they pass by the area of image capture, the system is unable to capture an image of the passing particles or it manages to capture an image in which the particles have an incorrect morphology. This has for example been observed for particles travelling with instant velocity varying between 3 and 20 m/s (the smaller the particle, the lower the threshold of instant velocity above which the particle is not captured in the image or it is captured with incorrect morphology).
Capturing undistorted images of objects moving at relatively high speed at the image capture area is a traditional problem. If the sampling rate of each pixel is not fast enough, the captured images will tend to be blurry and with shapes that do not correspond to reality. The sampling rate of each pixel depends mainly on two factors: the exposure time (the time for which each pixel collects the instant light that comes to it) and the reading time of each pixel (the maximum rate at which information from each pixel in the camera is transferred to a buffer). The capture of images on very dark oils can be compensated with an increase of the exposure time or with an increase of the intensity on the lighting source. While the exposure time is dependent on each application and is a function of the maximum illumination power available and of the opacity of the fluid to be analyzed, the reading time of each pixel depends on the technology of the video sensor.
Indeed, the problem of moving objects can be avoided by mechanical means using devices for flow regulation (in order to reduce the flow speed) or solenoid valves for stopping the oil sample under analysis. However, stopping or reducing the fluid flow rate directly impacts of the significance of the measurement, because samples are not being taken in real conditions. Flow regulators, such as needle valves for instance, tend to filter particles by themselves and can be easily stoppled by the accumulation of wear debris in highly contaminated fluids (like the ones targeted in the present invention) and the valves only allow the measurement of static samples with no flow at all, which impacts in the sampling speed and in the representativeness of the sample. Ideally, reducing the number of hydraulic elements enhances the measurement significance of an in-line or by-pass sensor.
Besides, certain techniques have been developed for overcoming the problem of capturing images of moving objects at the image capture area. One of the traditional techniques is called Rolling Shutter (RS). In RS, the pixels are activated sequentially one after the other. The temporal difference of the activation of a pixel with the forthcomings causes the captured images to appear moved and not focused if the objects move faster than the reading speed of each pixel. Another technique is called Global Shutter: This technology allows all pixels to be activated simultaneously, so regardless of the speed of the object to be captured, it appears static in the captured image. A third technique is called Rolling Shutter with Global Start (RSGS): This technology is a particular case of the Rolling Shutter, and allows all the pixels to be activated at the same instant, although the duration of the activation depends on the reading instant of each one, which is still sequential, which causes that some pixels are active longer than others.
However, in order to apply these techniques to the detection of microscopic particles comprised in a moving fluid, some parameters must be considered: (a) the resolution of the detector (height×width in pixels), (b) the pixel size and (c) the sensor area, because these three parameters determine (1) the minimum detectable size of a particle (which is dependent on the ratio between resolution and pixel size) and (2) the volume of analyzed fluid in each image (which is dependent on the sensor's active area). For this reason, Global Shutter techniques are discouraged for the detection of small particles in a moving fluid because in the state of the art solutions, due to their higher complexity in the pixel structure, they tend to offer lower resolutions and larger pixel sizes. Besides, even if Rolling Shutter solution is able to offer decent resolutions, pixel sizes for this specific application are not an option due to their limitations for capturing sharp images of moving objects. On the contrary, RSGS allows maintaining the advantages of the RS sensors while offering a performance in the capture of moving objects similar to the GS, under certain conditions, such as the isolation of CMOS detector of the non-controlled light sources (e.g. ambient lighting).
Nevertheless, when applying traditional RSGS setups to the detection of particles in a moving fluid, it has been observed that the system does not perform as expected for two reasons: (1) the image capture system does not receive enough light power (flash gain), which is normally required for the images and particles suspended on the fluid to have enough contrast in order to enable their discrimination; and (2) the time for which the lighting system is on (flash duration) is too elevated and therefore the images are not captured in a clear way.
Chinese patent application CN2899386 discloses a LED-array source for inspecting a printing image, in which the LED source provides a pulsed signal synchronized with the image printing speed. In order to synchronize the pulse signal with the printing speed, a circuit is designed for adjusting the frequency of the pulse signal. However, this disclosure does not give any clue regarding as how to provide very high power pulses from a conventional low current DC power source. Additionally, in systems for inspecting a moving fluid, the light pulses cannot be synchronized with the particle velocity because the inspection system does not know this velocity.
Therefore, there is a need for a system for inspecting a moving fluid which overcomes the former drawbacks.